1. Field of the Invention
This invention relates to an image sensor array using amorphous silicon photo-diodes, and more particularly to a structure for improving the resolution of a color image.
2. Description of the Prior Art
A contact type image sensor has been recently developed for use in facsimile equipment. Since the contact type image sensor can be formed in the same size as the document to be read, an optical lens system can be removed from the facsimile equipment to make the equipment compact.
As a semiconductor material of photo-diodes, amorphous silicon (hereinafter, referred to as .alpha.-Si) has been widely used in contact type image sensors. The .alpha.-Si was deposited on individual electrodes formed on an insulating substrate. Transparent electrodes were formed on the .alpha.-Si as common electrodes. The photodiodes were formed at the interface between the .alpha.-Si and the common electrodes by using a transparent metal which formed a Schottky junction with the .alpha.-Si as the common electrode. Alternatively, the photo-diodes were formed in the .alpha.-Si as P-N diodes or PIN diodes by introducing appropriate impurities into the .alpha.-Si. Color filters of red, green and blue are formed on parts of the common electrode positioned upon the individual electrodes. The red, green and blue filters are serially arranged in a line.
The photo carriers accumulated in the photo-diodes were sequentially read out from the individual electrodes. Such reading was performed by scanning switches which are respectively connected between a power source and the individual electrodes. An example of the switches is an MOS field effect transistor. One picture element of a color image was reproduced by using three outputs from photo-diodes positioned under the red, green and blue filters. Here, these three filters were arranged in a line. Therefore, the resolution of the reproduced picture was low, compared with a reproduced monochrome picture. Such low resolution may be improved by increasing the density of the photo-diodes. However, the increment of the density results in a minimization of each photo-diode. This causes a deterioration of the S/N ratio in output signals.
Such low resolution may alternatively be improved by arranging the photo-diodes and color filters in three parallel lines so as to form a red line, a blue line and a green line. However, in such a structure, all three sets of the photo-diodes and the color filters are formed on one individual electrode. Such structure cannot read out the photo carriers from a selected one of the photo-diodes. That is, the photo carriers accumulated in the other two photo-diodes which are formed on the same individual electrode cannot be prevented from being read out together. Thus, the reproducibility of color image may be poor.
H. Yamamoto et al proposed the use of a blocking diode connected in series with a photo diode in "Extended Abstracts of the 15th Conference on Solid State Devices and Materials" 1983, pages 205 to 208. The blocking diode and the photo diode are formed side by side on an insulating substrate and connected in series by wiring layers. All the blocking diodes and the photo diodes are arranged in lines. Therefore, the resolution in the reproduced picture was poor. Many complicated wirings and wide areas were required on the insulating substrate. This means the color image sensor could not be formed compactly.